Scala - 如何去解决“值不是Nothing的成员”错误 [英] Scala - how to go resolve "Value is not a member of Nothing" error

问题描述

此示例代码基于Atmosphere类，但如果有人可以给我一些错误的意义，我想我可以找出任何大气特定的解决方案...

  val bc = BroadcasterFactory.getDefault（）。lookup（_broadcasterId）
 bc.broadcast（message）
  
 
 
 
在第一行之后，bc应该包含一个对象的句柄，其类定义包括方法broadcast（） - 实际上，它包含几个重载的变体。但是，编译器使用以下代码阻塞第二行代码：value broadcast不是Nothing的成员
 
 
 
任何关于会导致什么的想法/建议？
 
 
 
谢谢。 
 
 
 
编辑：[BroadcasterFactor]的签名.lookup：
抽象广播者查找（Object id）
 
 
 
注意：1）这是我在示例中使用的签名版本，2）它是java Inteface签名 - 而getDefault（）回传实例化的对象实现该接口。 
 
 
 
解决方案：强制类型转换为值：
  val bc： Broadcaster = BroadcasterFactory.getDefault（）。lookup（_broadcasterId）
  
 
 
解决方案
  Nothing 是类型名称。它是所有其他类型的子类型。你不能从本身调用方法，你必须指定确切的类型（（bc：ExactType）.broadcast（message））。 没有任何没有实例。方法，返回 Nothing 将实际上从不返回值。 
 
 
 
  https://github.com/Atmosphere/atmosphere/blob/master/modules/cpr/src/main/java/org/atmosphere/cpr/BroadcasterFactory.java#L146 =nofollow> <$ c的定义$ c> lookup  ：
  abstract public< T extends Broadcaster> T lookup（Object id）;在scala中，
  
这个定义看起来是这样的：
  def lookup [T< ;: Broadcaster]（Object id）：T 
  
  lookup 方法中没有指定类型参数。在这种情况下，编译器会将此类型参数推断为最具体的类型 -   Nothing ：
  scala> def test [T]（i：Int）：T = ??? 
 test：[T]（i：Int）T 
 
 scala> lazy val x = test（1）
 x：Nothing =< lazy> 
 
 scala> lazy val x = test [String]（1）
 x：String =< lazy> 
  
您可以像这样指定类型参数：
  val bc = BroadcasterFactory.getDefault（）。lookup [Broadcaster]（_ broadcasterId）
  
 
 b $ b 
 实施草案 
 
 
 
在开发过程 lookup 实现像这样：
  def lookup（...）=？ 
  
   c> Nothing 。
 
 
 
您应指定 lookup 方法的结果类型： def lookup（...）：< TypeHere> = ... 或类型 bc ： val bc：< TypeHere> = 。
 
This example code is based on Atmosphere classes, but if someone could give me some insights into what the error means in general, I think I can figure out any Atmosphere-specific solution...
val bc = BroadcasterFactory.getDefault().lookup(_broadcasterId) 
bc.broadcast(message)
After the first line, bc should contain a handle to an object whose class definition includes the method broadcast() -- in fact, it contains several overloaded variations. However, the compiler chokes on the second line of code with the following: "value broadcast is not a member of Nothing"

Any ideas/suggestions on what would be causing this?

Thanks. 

EDIT: signature for [BroadcasterFactor].lookup :
    abstract Broadcaster lookup(Object id)

Note: 1) that is the signature version that I've used in the example, 2) it is the java Inteface signature - whereas the getDefault() hands back an instantiated object that implements that interface. 

Solution: force type cast on value:
val bc: Broadcaster = BroadcasterFactory.getDefault().lookup(_broadcasterId)

解决方案
Nothing is the type name. It's the subtype of all other types. You can't call methods from Nothing itself, you have to specify exact type ((bc: ExactType).broadcast(message)). Nothing has no instances. Method, that returns Nothing will, actually, never return value. It will throw an exception eventually.

Type inference

Definition of lookup:
abstract public <T extends Broadcaster> T  lookup(Object id);
in scala this definition looks this way:
def lookup[T <: Broadcaster](Object id): T
There is not specified type parameter in lookup method. In this case compiler will infer this type parameter as the most specific type - Nothing:
scala> def test[T](i: Int): T = ???
test: [T](i: Int)T

scala> lazy val x = test(1)
x: Nothing = <lazy>                                                                                                                                            

scala> lazy val x = test[String](1)                                                                                                                            
x: String = <lazy>
You could specify type parameter like this:
val bc = BroadcasterFactory.getDefault().lookup[Broadcaster](_broadcasterId) 
Draft implementation

In development process lookup can be "implemented" like this:
def lookup(...) = ???
??? returns Nothing.

You should specify either result type of lookup method like this: def lookup(...): <TypeHere> = ... or type of bc: val bc: <TypeHere> =.

                        
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